In one type of nuclear reactor referred to as an advanced liquid metal reactor (ALMR) a nuclear reactor core is submerged in a hot liquid metal such as liquid sodium within a reactor vessel. The liquid metal is used for cooling the reactor core, with the heat absorbed thereby being used for producing power in conventional manners. Surrounding the reactor vessel is a containment vessel, with the space therebetween being filled with an inert gas such as argon.
Operation of the reactor is controlled by control rods which are selectively inserted into or withdrawn from the reactor core. The control rods may be fully inserted therein in order to shutdown the reactor core. However, residual decay heat continues to be generated from the core for a certain time, with the heat being transferred by thermal radiation from the reactor vessel to the containment vessel which increases its temperature. heat from the containment vessel will also radiate outwardly toward a concrete silo spaced outwardly therefrom. In order to prevent excessive heating of these components, a passive heat removal system referred to as the reactor vessel auxiliary cooling system (RVACS) is provided and is disclosed in U.S. Pat. No. 5,043,135 for example, which is assigned to the present assignee.
In the current RVACS, an imperforate heat collector cylinder is disposed concentrically between the containment vessel and the silo to define an air riser between its inner surface and the containment vessel, and an air downcomer between its outer surface and the silo. Atmospheric air is suitably channeled downwardly through the downcomer to its bottom wherein it is turned upwardly into the air riser for flow upwardly to cool the containment vessel. The inner surface of the collector cylinder receives thermal radiation from the containment vessel, with the heat therefrom being transferred by natural convection into the rising air for flow upwardly to remove the heat. The outer surface of the collector cylinder includes thermal insulation to reduce transfer of the heat from the collector cylinder into the silo and into the air flowing downwardly in the downcomer. The greater the differential in temperature between the relatively cold downcomer air and the heated air within the riser, the greater will be the degree of natural circulation for driving the air cooling passively without motor-driven pumps.
In this configuration, the average temperature of the containment vessel during steady-state operation as well as the transient peak temperatures thereof following certain transient operations, are relatively high, which requires that the containment vessel be designed to high-temperature ASME code requirements, which increases the cost thereof. Furthermore, the thermal insulation provided over the outer surface of the heat collector cylinder is complex and relatively expensive to ensure that the concrete silo is not excessively heated. Accordingly, improved air cooling of the containment vessel is desired for reducing complexity and cost of the cooling system.